MAX3669(1999) View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX3669
(Rev.:1999)

+3.3V, 622Mbps SDH/SONET Laser Driver with Current Monitors and APC

Maxim Integrated 

+3.3V, 622Mbps SDH/SONET

Laser Driver with Current Monitors and APC

Calculate Power Consumption

The total power dissipation of the MAX3669 can be esti-

mated by the following:

P = VCC · ICC + (VCC - Vf) · IBIAS

+ IMOD (VCC - 20Ω · IMOD / 2)

where IBIAS is the maximum bias current set by RBIAS-

MAX, IMOD is the modulation current, and Vf is the typi-

cal laser forward voltage.

Applications Information

The following is an example of how to set up the

MAX3669.

Select Laser

A communication-grade laser should be selected for

622Mbps applications. Assume the laser output aver-

age power is PAVG = 0dBm, the minimum extinction

ratio is re = 6.6 (8.2dB), the operating temperature is

-40°C to +85°C, and the laser diode has the following

characteristics:

Wavelength:

λ = 1.3µm

Threshold Current:

Threshold Temperature

Coefficient:

Laser to Monitor Transfer:

Laser Slope Efficiency:

ΙTH = 22mA at +25°C

βTH = 1.3%/°C

ρMON = 0.2A/W

η = 0.05mW/mA

at +25°C

Determine RAPCSET

The desired monitor diode current is estimated by

IMD = PAVG · ρMON = 200µA. The Monitor Diode Current

vs. APC Set Resistor graph in the Typical Operating

Characteristics shows that RAPCSET should be 6kΩ.

Determine RMODSET

To achieve a minimum extinction ratio (re) of 6.6dB over

temperature and lifetime, calculate the required extinc-

tion ratio at +25°C. Assuming re = 20, the peak-to-peak

optical power Pp-p = 1.81mW, according to Table 1.

The required modulation current is 1.81(mW) /

0.05(mW/mA) = 36.2mA. The Modulation Current vs.

Modulation Set Resistor graph (see Typical Operating

Characteristics) shows that RMODSET should be 5kΩ.

Determine RBIASMAX

Calculate the maximum threshold current (ITH(MAX)) at

TA = +85°C and end of life. Assuming ITH(MAX) =

50mA, the maximum bias current should be:

IBIAS = ITH(MAX) + IMOD / 2

In this example, IBIAS = 68.1mA. The Bias Current vs.

Maximum Bias Set Resistor graph in the Typical

Operating Characteristics shows that RBIASMAX should

be 3kΩ.

Determine RBIASMON

To avoid saturating the current mirror of BIASMON, the

voltage at this pin should not drop below (VCC - 1.6V).

The resulting condition is:

RBIASMON

≤



1.6V

ABIAS

IBIASMAX





where IBIASMAX is the maximum current expected for

the application.

Determine RMODMON

To avoid saturating the current mirror of MODMON, the

voltage at this pin should not drop below (VCC - 1V).

The resulting condition is:

RMODMON

≤



1V

AMOD

IMOD





Modulation Currents Exceeding 50mA

To drive modulation currents greater than 50mA at

3.3V, external pull-up inductors (Figure 5) should be

used to DC-bias the modulation output at VCC. Such a

configuration isolates the laser forward voltage from the

output circuitry and allows the output at OUT+ to swing

above and below the supply voltage VCC.

VCC

10Ω

OUT-

OUT+

MAX3669

BIAS

MD

FERRITE

LD

BEADS

CD RD

1µF 5Ω

FERRITE BEAD

1000pF

RFILT

CFILT

Figure 5. Output Termination for Maximum Modulation Current

10 ______________________________________________________________________________________

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